Added SETURIPRESENT and SETURIMISSING to external special remote protocol

Useful for things like ipfs that don't use regular urls.

An external special remote can add a regular url to a key, and then
git-annex get will download it from the web. But for ipfs, we want to
instead tell git-annex that the uri uses OtherDownloader. Before this
change, the external special remote protocol lacked a way to do that.

Remote/External.hs

@@ -223,6 +223,10 @@ handleRequest' lck external req mp responsehandler
 		setUrlPresent (externalUUID external) key url
 	handleRemoteRequest (SETURLMISSING key url) =
 		setUrlMissing (externalUUID external) key url
+	handleRemoteRequest (SETURIPRESENT key uri) =
+		withurl (SETURLPRESENT key) uri
+	handleRemoteRequest (SETURIMISSING key uri) =
+		withurl (SETURLMISSING key) uri
 	handleRemoteRequest (GETURLS key prefix) = do
 		mapM_ (send . VALUE . fst . getDownloader)
 			=<< getUrlsWithPrefix key prefix
@@ -242,6 +246,9 @@ handleRequest' lck external req mp responsehandler
 		}
 	  where
 		base = replace "/" "_" $ fromUUID (externalUUID external) ++ "-" ++ setting
+			
+	withurl mk uri = handleRemoteRequest $ mk $
+		setDownloader (show uri) OtherDownloader
 
 sendMessage :: Sendable m => ExternalLock -> External -> m -> Annex ()
 sendMessage lck external m = 

Remote/External/Types.hs

@@ -43,6 +43,7 @@ import Utility.Url (URLString)
 import qualified Utility.SimpleProtocol as Proto
 
 import Control.Concurrent.STM
+import Network.URI
 
 -- If the remote is not yet running, the ExternalState TMVar is empty.
 data External = External
@@ -182,6 +183,8 @@ data RemoteRequest
 	| GETSTATE Key
 	| SETURLPRESENT Key URLString
 	| SETURLMISSING Key URLString
+	| SETURIPRESENT Key URI
+	| SETURIMISSING Key URI
 	| GETURLS Key String
 	| DEBUG String
 	deriving (Show)
@@ -202,6 +205,8 @@ instance Proto.Receivable RemoteRequest where
 	parseCommand "GETSTATE" = Proto.parse1 GETSTATE
 	parseCommand "SETURLPRESENT" = Proto.parse2 SETURLPRESENT
 	parseCommand "SETURLMISSING" = Proto.parse2 SETURLMISSING
+	parseCommand "SETURIPRESENT" = Proto.parse2 SETURIPRESENT
+	parseCommand "SETURIMISSING" = Proto.parse2 SETURIMISSING
 	parseCommand "GETURLS" = Proto.parse2 GETURLS
 	parseCommand "DEBUG" = Proto.parse1 DEBUG
 	parseCommand _ = Proto.parseFail
@@ -288,3 +293,7 @@ instance Proto.Serializable [(URLString, Size, FilePath)] where
 	  where
 		go c (url:sz:f:rest) = go ((url, readish sz, f):c) rest
 		go c _ = reverse c
+
+instance Proto.Serializable URI where
+	serialize = show
+	deserialize = parseURI